“The internet is fundamentally a system designed to share information.

“And child [abuse] is information – I know it is also a lot more than that – but to a computer it is just information.”

He said even if Cameron’s plans were successful people would “just start mailing CDs to one another”.

“Then you would have to shut down Royal Mail as well,” he said.

Freenet is not the only dark net system. Another is Tor, which was created by the US Navy.

Seth Schoen, of the Electronic Frontier Foundation, which fights for internet access, said: “Tor hidden services are internet services that use a feature of the Tor privacy network to hide the physical location of the server that is operating the service.

“So it’s much harder to find who is running the service or where they’re running it from.”

Unlike normal internet services that may promise privacy, privacy is designed into Tor.

“Tor hidden services are more resistant to some kinds of censorship because it’s difficult to exert any kind of pressure on their anonymous operators,” Mr Schoen said.

“Unlike connecting to an ordinary internet service, it’s not clear to your internet service provider what service you’re using or whether you’re using a hidden service at all.”

Search engines do not commit an offence if criminal material can be accessed through their technology.

“If they are not involved themselves and are just providing a platform then they will not be in trouble,” said media and internet lawyer Rupinder Bains.

“They would have to remove anything illegal if they were informed.”

But the operators of systems like Tor cannot tell who is behind illegal sites.

Ms Bains warned there was a lack of “police training and resources and a lack of knowledge.”

“There is really no answer at the moment,” she said.

“There needs to be a great shake up of the law to give police greater powers.

“The law is not completely up to speed with the technological age. It is not keeping pace.”

Jim Killock, executive director of civil liberties campaigners the Open Rights Group, said Cameron’s proposals were the “least effective measure he could take to solve the problem”.

“What he really needs to do is give money to the police. He needs to challenge foreign governments who are taking too long to remove websites that sell child abuse images and he needs to tackle money laundering so that gangs find it difficult to make money out of the sale of child abuse images,” he said, adding that targeting Google was a “cheap shot”.

“You can’t stop it because you would have to ban anyone from running Tor,” he said.

“And banning a technology is an extremely difficult thing to do. Especially when it is aiding freedom of speech activists.”

Police would have to bring down paedophiles individually, he said.

“If people are using technology for illegal reasons it is possible for police to spend police time infiltrating networks through such mechanisms.

“That’s what paedophiles do and it ought to be in the reach of the police.”

A Child Exploitation and Online Protection Centre source said it was “harder than you think” to find child abuse on Google – though he thought it could be found.

“People think you just type in ‘child porn’ and a lot of research shows that is not the case. If you do you will find a lot of news stories.”

The insider recognised the “professional paedophile” would not use traditional search engines to access child abuse.

But someone curious about abuse might be deterred by warnings they were about to access illegal content.

“Part of what we do day to day is we scan and monitor how they contact each other for images,” the source said.

“The way they talk to each other, they almost use their own language. We can create a blacklist of terms.

“If you read them you might think ‘What the hell does that mean?’

“That in effect is how they would look for it. They would put one of these terms into Google and then in effect they would have to do a bit of work.”

They may have to follow hundreds of links to get near what they want.

“That is when the paedophile world opens up to them and sends them off down all different avenues,” the insider said.

“Warning messages that came up would help stop the curious child abuser. It might make them think twice that what they were doing was wrong.

“But for those past the barrier that is not going to work for them.”

File sharing among perverts was another issue.

“How are we going to tackle that?” said the source.

“There is a lot to be discussed and we are going to be working with the industry to see what ideas they have got.”

A Downing Street spokeswoman insisted Cameron was going to take on child abuse on the dark net.

“Whenever we discover it, we respond quickly to remove and report it. We recently donated $5m to help combat this problem and are committed to continuing the dialogue with the government on these issues.”

We did not view any images of child abuse in researching this article.

What is the dark net?

Dark nets are secretive networks where connections are made between trusted peers – like on Facebook, these are sometimes called friends.

The unique number, or IP address, each computer has in a network is not shared when dark nets are used.

This means users can communicate anonymously, without fear of political interference.

The sites are entirely legal but this anonymity has allowed black market activity to flourish.

While some publish on dark nets to avoid political reprisal, others use them for criminal gain.

They were developed in the 1970s for security purposes. They were isolated from the Arpanet, which went on to become the internet, but they could receive data from it.

CIA whistleblower Edward Snowden is a fan of dark nets – he sported a Tor sticker on his laptop.

On that site users do not know who is behind an internet site and site creators do not know who they are being viewed by.

Dark nets aside from Tor include Freenet, Gnunet and I2P.

Like the early internet, information can be hard to find.

This is not helped by the fact that links are often jumbles of letters and numbers. On Tor they may not feature words as addresses do on the mainstream web.

According to one blogger: “The first rule of the dark net is that you do not talk about the dark net.”

What Cameron said

“Our collective lack of action on the internet has led to harmful and, in some cases, truly dreadful consequences for children.

“In Britain, Google, Microsoft and Yahoo are already actively engaged on a major campaign to deter people who are searching for child abuse images. Companies like Google make their living out of trawling and categorising content on the web, so that in a few key strokes you can find what you’re looking for.

“We need a situation where you cannot have people searching for child abuse images and being aided in doing so. If people do try and search for these things, they are not only blocked, but there are clear and simple signs warnings that what they are trying to do is illegal.

“What we’ve already done is insist that clear, simple warning pages are designed and placed wherever child abuse sites have been identified and taken down so that if someone arrives at one of these sites they are clearly warned that the page contained illegal images.

“There are some searches which are so abhorrent and where there could be no doubt whatsoever about the sick and malevolent intent of the searcher.

“In these cases, there should be no search results returned at all. There needs to be a list of terms – a blacklist – which offer up no direct search returns.

“Google, Bing, Yahoo! and the rest: you have a duty to act on this, and it is a moral duty.

“I can tell you we’re already looking at legislative options so that we can force action in this area.

“The cultural challenge is the fact that many children are watching online pornography and finding other damaging material online at an increasingly young age.

“There’s been a big debate about whether internet filters should be set to a default ‘on’ position, in other words with adult content filters applied by default, or not.

“We need good filters that are pre-selected to be on, pre-ticked unless an adult turns them off, and we need parents aware and engaged in the setting of those filters.”

The Silk Road

At a glance it could be mistaken for eBay.

But notorious dark net website Silk Road sells heroin, crack, cocaine and any other drug you can think of.

Like eBay there are lists of what is on offer and there is even buyer feedback.

One seller had 3.5g of “Mexican black tar heroin” for sale.

“Can’t make any claims on quality as we’re not users of this particular product, though I can say local users find it agreeable,” the dealer said.

Another offered uncut crack and promised to post within 24 hours of an order being placed.

“I am aware of the latest stealth packaging,” the seller said, adding that he was happy to ship worldwide.

Buyers did not have to worry about their address getting into the wrong hands.

“I destroy all your information after the order has been furnished,” said the dealer.

One customer was very pleased with the service giving him a five star rating.

“Professional and reliable,” the shopper said.

Another praised the “super fast delivery to the UK”.

“Very good crack,” said a third.

The site used to offer guns for sale as well as drugs. This was stopped when it proved unpopular with some users.

So a separate armoury site was opened. That closed because of sluggish sales.

Freenet founder Ian Clarke knew about the site.

“The Silk Road is like eBay for drugs,” he said.

“I have looked at it and it is surprisingly like eBay.

“You can review the website and give the drug dealers up to five stars to keep them honest.

“You buy the drugs and they are then mailed to you.”

Payment is made with Bitcoins, an untraceable virtual currency.

Last night one bitcoin was worth about £62.

“They are essentially like e-gold,” Ian said.

“Gold that you can distribute over the net very conveniently and like gold are scarce. They have to be ‘mined,’ just like gold.”

Authorities around the globe are desperate to catch up with the those behind Silk Road.

The founder’s identity and location are a mystery.

He goes by the name of “The Dread Pirate Roberts”. The pseudonym is taken from William Goldman’s novel The Princess Bride.

In that story Roberts is not one man, but a series of individuals who pass the name and ruthless reputation to a chosen successor.

The implication is that Roberts of Silk Road cannot be stopped.

The underworld entrepreneur seemed unworried by his status as one of the world’s most wanted, and unconcerned about getting caught.